This invention relates to sheet silicates, also known as lattice layered silicates or phyllosilicates. It is particularly concerned with solid solutions produced from phyllosilicates, and with an ion exchange procedure for their production.
Essentially any phyllosilicate, whether of natural or synthetic origin, may serve as a starting material. However, the natural materials are generally of greater interest from an economic standpoint. The silicate minerals of interest include vermiculite, beidellite, nontronite, volchonskoite, saponite, stevensite, sauconite, pimelite, bentonite, montmorillonite, hectorite, the smectites, attapulgite, sepiolite, phlogopite and biopyrobole.
Sheet silicates of the mica type are built of two units, viz., a tetrahedral sheet and an octahedral sheet. The former consists of tetrahedra of Si--O linked together to form a hexagonal network such that the bases thereof are coplanar and the apices thereof point in the same direction. This configuration yields a Si:O ratio of 2:5. In contrast, the octahedral sheet is generated through the impingement of two tetrahedral sheets pointing toward each other and crosslinked by the sharing of oxygens by Mg (or Al, Fe) in octahedral coordination. The two octahedral corners not falling in the plane of apical oxygens are occupied by hydroxyl or fluoride ions. It is possible that a composite sheet formed in this manner will be electrically neutral, in which case Van der Waals-type forces bond it to the sheets immediately above and below. More commonly, however, an excess negative charge exists due either to ion substitutions or unoccupied sites (vacancies) or a combination of both. Differences in properties arise both from the degree of charge deficiency as well as the location of the excess charge. Charge balance is restored through the uptake of foreign cations in interlayer positions in 12-fold coordination due to hexagonal rings of oxygens located in the sheets above or below.
In order to create a product from vermiculite, it is usually necessary to delaminate the particles. This involves separating the crystals at the interlayer to form high aspect ratio platelets. These may be suspended as a gel and subsequently deposited in any desired form, such as a sheet, or otherwise processed.
At one time it was standard practice to heat vermiculite particles to an elevated temperature. This caused the water-containing interlayer to expand and pop open. Later, it was learned that vermiculite could be expanded by reflux treatment with various salts in aqueous solution. Thereafter, application of an intense shearing force to the expanded particles caused them to separate at the interlayer and form a gel.
The silicate layer units in these minerals have a thickness of about 10 Angstrom units, with the main elemental constituents being Mg, Al, Si, and O.sub.2. These silicate layers are separated by an interlayer composed of water molecules associated with cations, such as Mg.sup.++, Na.sup.+, K.sup.+ and H.sup.+.
The three layer micas in general, and natural vermiculite in particular, have been extensively studied because of their potential for thermal resistance and electrical insulation. The interest has been heightened considerably with the recent flight from asbestos products.
There are areas of utility where it would be desirable to modify the normal properties of the phyllosilicates. For example, a lower coefficient of expansion would be desirable in some instances. Likewise, iron impurities may be deleterious to electrical properties, and removal would be expedient. Further, greater resistance to rehydration would be a boon.